Functional consequences of mutations in the smooth muscle myosin heavy chain at sites implicated in familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) is frequently associated with mu tations in the beta-cardiac myosin heavy chain. Many of the implicated resi dues are located in highly conserved regions of the myosin II class, sugges ting that these mutations may impair the basic functions of the molecular m otor. To test this hypothesis, we have prepared recombinant smooth muscle h eavy meromyosin with mutations at sites homologous to those associated with FHC by using a baculovirus/ insect cell expression system. Several of the heavy meromyosin mutants, in particular R403Q, showed an increase in actin filament velocity in a motility assay and an enhanced actin-activated ATPas e activity. Single molecule mechanics, using a laser trap, gave unitary dis placements and forces for the mutants that were similar to wild type, but t he attachment times to actin following a unitary displacement were markedly reduced. These results suggest that the increases in activity are due to a change in kinetics and not due to a change in the intrinsic mechanical pro perties of the motor. In contrast to earlier reports, we find that mutation s in residues implicated in FHC affect motor function by enhancing myosin a ctivity rather than by a loss of function.